The present invention relates to an electrophotographic process and more particularly, to a method of adjusting photosensitivity of a photosensitive member or photoreceptor in an electrophotographic process. More specifically, the present invention relates to a method of photosensitivity adjustment of a photosensitive member having peculiar hysteresis characteristics, i.e., a phenomenon in which, when the photosensitive member is subjected to previous processing such as corona charging, exposure to light etc., photosensitivity of the photosensitive member thereafter varies in correspondence with such previous processing as is observed in a photosensitive member which is composed of photoconductive fine particles of cadmium sulfide (CdS) and cadmium carbonate (nCdCO.sub.3) (0&lt;n.ltoreq.4) dispersed in a binder resin together with metallic active agent (referred to as a CdS.nCdCO.sub.3 -resin photosensitive member hereinbelow).
As a result of studies concentrated on the CdS.nCdCO.sub.3 -resin photosensitive member, the present inventors have found in such a photosensitive member, hysteresis characteristics different from those in other ordinary photosensitive members. More specifically, in the hysteresis characteristics of the ordinary photosensitive member exhibiting fatigue phenomenon, when the photosensitive member is subjected to light projection of high intensity illumination, the electrical charge acceptance capacity of the photosensitive member is reduced to such an extent that the photosensitive member is unable to be used as a photosensitive member unless it is kept in a dark place for a predetermined period of time for restoration of its charge acceptance capacity. On the contrary, the CdS.nCdCO.sub.3 -resin photosensitive member does not exhibit light projection of fatigue even when exposed to light projection of high intensity illumination and shows good reproducibility repeatedly, with photosensitivity thereof not being affected by such light projection illumination, but upon further charging and exposure of the photosensitive member as previous processing for the subsequent process, the photosensitivity of the photosensitive member thereafter becomes variable, with the degree of variability being altered by the amount of the hysteresis exposure. Such peculiar characteristics of the CdS.nCdCO.sub.3 -resin photosensitive member are shown in FIG. 1, in which samples a, b, c, d and e of the CdS.nCdCO.sub.3 -resin photosensitive member (n.apprxeq.1) of 40 .mu. thick were each sequentially subjected to light projecting processes such as a process equivalent to light projection of more than 1,000 lx.sec., hysteresis charging, hysteresis exposure, charging, and exposure to image-wise light. In the hysteresis charging and the charging, each of the samples a to e was charged up to a surface potential of 1,000 V, while the amount of exposure in the hysteresis exposure was varied to be 10.sup.-1 lx.sec. in the sample a, 10.sup.0 lx. sec. in the sample b, 10.sup.1 lx.sec. in the sample c, 10.sup.2 lx. sec. in the sample d, and 10.sup.3 lx.sec. in the sample e, to observe light attenuation in the light projection corresponding to the exposure to image-wise light. As is seen from corresponding curves a to e in FIG. 1, the photosensitivity at the exposure to image-wise light is varied according to the amount of the hysteresis exposure. Referring also to FIG. 2 showing the above relation more specifically and employing as a measure of the photosensitivity during the exposure to the image-wise light, the reciprocal of the amount of exposure required to reduce the surface potential of the charged photosensitive member to half, it is noticed that the photosensitivity reaches a peak value at a certain amount of exposure P, and that the photosensitivity increases as the amount of exposure increases in a region below the amount of exposure P, while in a region above the amount of exposure P, the photosensitivity decreases as the amount of exposure increases.
As a fundamental image forming method for the photosensitive member having such a peculiar hysteresis characteristics described above, there has conventionally been proposed one method, for example, in U.S. patent application Ser. No. 834,972 by the present inventors, which method is described hereinbelow with reference to FIG. 3. In FIG. 3, around a photosensitive drum 1 in the direction of its rotation indicated by the arrow, there are sequentially disposed a lamp 2, a hysteresis charger 3 for the hysteresis charging, a hysteresis exposure lamp 4 for the hysteresis exposure, a corona charger 5, and an optical system 6 for projecting light image corresponding to the image of an original (not shown) to be copied onto the surface of a photosensitive member 1a provided around the photosensitive drum 1. The electrostatic latent image formed thereby may be transferred to the paper directly or through the developing process. It should be noted here that the lamp 2 has functions not only of erasing residual electrical charge on the photosensitive member as in the conventional erasers, but also of removing the effect due to the previous processings of the photosensitive member so that the photosensitive member is not influenced thereafter by such previous processing, and should be clearly distinguished in its functions from the conventional erasers. The lamp 2 has its intensity of illumination set at more than 1,000 lx.sec. for erasing the effects of the previous processing, while the hysteresis charger 3 is adapted to impart charge equal to or more than the surface potential imparted by the charger 5. Meanwhile, the hysteresis exposure lamp 4 is adapted to project light of approximately 10.sup.1 lx.sec. onto the photosensitive member 1a so that the photosensitive member 1a can be used at its portion having the highest photosensitivity so as to correspond to the curve c of FIG. 1. The image forming method as described above has made it possible to actually use the photosensitive member having the peculiar hysteresis characteristics such as the CdS.nCdCO.sub.3 -resin photosensitive member which has not been usable in the ordinary image formation due to the influence of the previous processing accompanied by undesirable fogging, memory effect, etc., and is arranged to subject the photosensitive member 1a to previous processing so that the photosensitive member has the highest photosensitivity through utilization of the hysteresis characteristics of said photosensitive member.
An essential object of the present invention is to provide an improved electrophotographic process in which photosensitivity adjustments of the photosensitive member are effected for (1) control of reproduction density in a copied image, (2) supplementing brightness or light amount at edge portions of an image field on the photosensitive member, and (3) improvement of gradation reproducibility in the copied image through meshwork resolving function i.e., breaking up of the image into numerous dots.
Another important object of the present invention is to provide an electrophotographic process as described above which is readily applicable to electrophotographic copying apparatuses for incorporation thereinto at low cost.
In accomplishing these and other objects, according to the present invention, the electrophotographic method of copied image formation including the steps of sequentially subjecting photosensitive member whose photosensitivity during exposure thereof to light images varies depending on exposure the amount of light in previous processing, to preexposing step, hysteresis charging process, hysteresis exposure process, charging process and light image exposure process further includes either one or combination of the following three steps:
(1) adjusting the photosensitivity of the photosensitive member by making the amount of light in the hysteresis exposure process uniformly variable in the axial direction of said photosensitive member.
(2) making the photosensitivity at peripheral or edge portions of the photosensitive member larger than that at the central portion of the photosensitive member by selectively increasing or decreasing the amount of light at the peripheral portion of the photosensitive member in the hysteresis exposure process with respect to the amount of light at the central portion of said photosensitive member.
(3) forming a meshwork or dot pattern on the photosensitive member resulting from variation of the light sensitivity through variation in the form of meshwork of the amount of light in the hysteresis exposure process.
By the arrangement as described above, adjustment of contrast in the copied image can be made simultaneously with the reproduction density control, while the insufficiency of amount of light at edge portions on the photosensitive member is eliminated, with a favorable tone gradation reproducibility being achieved through the meshwork resolving function.